LCD (liquid crystal display) and EL (electroluminescence display) are in their development or commercial stage, as image display devices of so-called flat panel type. Since the liquid crystal display is thin and lower in consumption power, it is used for monitors for personal computers and various information processing units, or television receiver sets. The electroluminescence display on the other hand is light emitting type, which does not require any external light source used for example in the liquid crystal display devices, allowing making thinner and light weight image display devices. The active matrix image display of such devices uses an active matrix substrate having a number of pixel circuits of matrix arrangement, active elements that drives the matrix circuits, pixel driver circuit that drives pixel circuits, and any other auxiliary circuit necessary for image display formed on a same dielectric substrate. The pixel circuits and pixel driver circuits uses as circuitry elements active elements formed on a silicon semiconductor layer formed on the dielectric substrate constituting the active matrix substrate. The exemplary active element is thin film transistors.
Recently, adopting thin film transistors using low temperature polysilicon semiconductor film for those active elements to form the pixel driver and auxiliary circuits by the thin film transistor on a dielectric substrate to form an active matrix substrate is realized so as to obtain an image display device with high definition image and low manufacturing cost. Herein below low temperature polysilicon semiconductor film will be referred to as polysilicon film, or sometimes semiconductor film.
A widely used manufacturing method of semiconductor thin film of the type represented by the polysilicon semiconductor film may be laser crystallization method, conventionally consisted of forming an amorphous silicon semiconductor film on a dielectric substrate, exposing the amorphous silicon semiconductor film with laser beam and annealing and crystallizing. In particular in order to obtain a polysilicon semiconductor film on a large dielectric substrate such as the active matrix substrate used in the image display device for television receiver set, crystallization of multi-shot method or multi-shot laser crystallization method is used, in which a plurality of pulse excimer laser for yielding a higher output power is exposed for crystallization.
Multi shot laser crystallization method may yield a relatively large grain silicon crystal of the grain size of 0.5 μm or more. This may be useful for forming a semiconductor film having a high electron mobility suitable for the driver circuit contained in the active matrix substrate of an image display device. To obtain a uniform film of large size, it is common that the exposure pattern of the laser beam on the semiconductor film should be shaped to a rectangular, more specifically to a line beam in which the short axis width is extremely short with respect to the long axis width, and the dielectric substrate having semiconductor films formed thereon will be moved in relation to the short axis direction of the shaped beam during the exposure.
In the crystallization method of silicon thin film by the exposure of laser beam, narrowing the transfer distance of the laser beam during the exposure intervals between two laser pulses much smaller than the beam width (that is, the length of short axis) may obtain a good result of annealing effect. By uniformly forming a laser beam of line beam shape that has a long width in the long axis direction with respect to the width in the short axis direction, crystals may be grown with no gaps for a large area. Also, as an alternative method of obtaining a semiconductor film of high electron mobility with a larger grain size, crystallization using lateral growth is now being considered.
In the non-patent document 1, Japanese Journal of Applied Physics Vol. 31, (1992) pp. 4545-4549, it is disclosed that a larger grain crystal may be formed by exposing laser beams on an amorphous silicon semiconductor film while having the thermal capacity of the dielectric substrate differed to form a thermal gradient therein to cause growth of silicon crystals from the area of lower temperature to the area of higher temperature.
In the patent document 1, JP-A No. 140323/1994, it is disclosed a method that enlarges the grain size by modulating the excimer laser beam with a grating to expose an amorphous silicon semiconductor film with such a modulated beam to cause a temperature gradient and growth of crystals from the area of lower temperature to the area of higher temperature.
In the patent document 2, JP-A No. 274088/2001, it is disclosed a method that a sequential exposure of laser beam with translation of substrate of slight overlap to the previous melt zone to induce sequential growth of crystal in the lateral direction to form a large grain crystal, or it is referred to as SLS method. Another example of sequential exposure of pulse laser beam with a slight overlap to the previous melt zone is disclosed in the non-patent document 2, in which the pulsed laser beam is scanned while exposure on the amorphous silicon film. In this method it is described that the scan speed of the laser beam is 99 centimeters per second, and the frequency of laser beam is 2 kHz. The interval of 49.5 μm between two laser beam exposures may be calculated. In FIG. 9 of this reference it is described an example in which the melt width by each pulse of laser beam is more than 50 μm and a melting zone is overlapped to its previous melted zone.
In the patent document 3, JP-A No. 280302/2002 it is disclosed that a lateral growth method for forming a large grain crystal by exposing a laser beam of intensity modulated by the interference of laser beams while translating by the growth distance in the lateral direction. Another example of lateral grain growth method using laser modulation is disclosed in the non-patent document 3. In this paper, a phase shift mask, which is placed on the amorphous Si film substrate, modulate laser beam intensity periodically to induce lateral grain growth.
[Non-patent Document 1]
Japanese Journal of Applied Physics Vol. 31, (1992) pp. 4545-4549
[Non-patent Document 2]
Japanese Journal of Applied Physics Vol. 21, (1982) pp. 879-884
[Non-patent Document 3]
Japanese Journal of Applied Physics Vol. 37, (1998) pp. 5474-5479
[Patent Document 1]
JP-A No. 140323/1994
[Patent Document 2]
JP-A No. 274088/2001
[Patent Document 3]
JP-A No. 280302/2002
In order to improve the throughput when forming a large grain crystal by the above mentioned multi-shot laser crystallization for a semiconductor film formed on a dielectric substrate for use in an active matrix substrate of an image display device, it is imperative to reduce the number of shots of the laser beam. However, if the number of shots of laser beam is reduced when using a laser beam of uniform intensity without any modulation according to the prior art, the grain size of yielded crystal will be shrunk, resulting in a decrease of electron mobility, so that the improvement of throughput has been difficult.
On the other hand, in the semiconductor film crystallization method using the lateral growth, it is possible to decrease the number of shots of laser beam. However, in any conventional methods the relative position of laser beam with respect to the dielectric substrate should be controlled, by the length similar to the growth distance in the lateral direction. The lateral growth distance of a crystal may depend on the silicon film thickness, substrate temperature at the crystallization, and the pulse duration time of irradiated laser beam. For example when melting and crystallizing a silicon semiconductor substrate of film thickness of 50 nm with pulsed excimer laser beam of 25 nsec at the room temperature, lateral growth distance of crystals will be 1 μm or less. Therefore, the exposure position is required to be controlled at the precision of 1 μm or more. This requires a highly accurate translating mechanism, resulting in a higher installation cost.